It is known that certain urethanyl coating materials can be cured thermally or by radiation in the presence of a free radical photoinitiator but these materials do not lend themselves to cationically induced polymerization. It is well recognized that thermal curing is not cost efficient and that radiation curing in free radical systems is oxygen inhibited, thus requiring an inert atmosphere or the minimizing affect of a hydrogen donating component. The later expedient is not completely satisfactory since such hydrogen donating components significantly reduce the rate of reaction. Also, it has been found that polymerization or curing in free radical systems ceases almost immediately upon removing the source of radiation; thus, the cured product often contains significant amounts of unpolymerized components. Accordingly, it is an aim of research to develop monomers or oligomers which provide stable polymerizable formulations with urethane containing materials while incorporating other beneficial properties in the finished cured product. Additionally, it is desirable that such monomers or their oligomers be amenable to radiation curing at a rapid rate under mild temperature conditions by cationically induced polymerization which is not oxygen inhibited and which permits continued polymerization after removal from the source of radiation exposure.
One such group of compounds is the curable propenyl ether resins of U.S. Pat. No. 5,153,237 which may contain a propenyl ether group indirectly bonded to the internal backbone of the compound. However, a higher degree of flexibility, hardness and cross-link density than these compounds possess is desirable.
Accordingly, it is an object of the present invention to overcome the disadvantages of previous urethanyl prepolymers and to provide a novel urethanyl prepolymer of high vinyl active functionality which is cationically curable at room temperature by radiation.
Another object is to provide a urethanyl prepolymer containing many crosslinking sites directly bonded to a polymer backbone which is rapidly cured to a high density material under mild conditions.
Another object of this invention is to provide a convenient process for the preparation of the present prepolymer.
Still another object of this invention is to provide a protective coating with a cured high density prepolymer having improved hardness, flexibility, resistance to abrasion and chemical attack.
These and other objects will become apparent from the following description and disclosure.